The present invention relates generally to a viscous coupling apparatus of the type used in motor vehicles and, more particularly, to an improvement in the viscous coupling plates utilized within the viscous coupling apparatus.
Viscous couplings are used in various applications within the drivetrain of a motor vehicle. In one typical application, a viscous coupling is located within the transfer case of a four-wheel drive vehicle to avoid tight corner braking by allowing slip to occur between the front and rear axles while concomitantly delivering torque to the wheel with traction. In another vehicular application, the viscous coupling is used in association with a differential to secure effective power delivery and improve traction, handling and stability. In still another application, the viscous coupling is a viscous transmission device that is located between the front and rear axles to provide "on-demand" four-wheel drive operation.
In general, viscous couplings include of a rotatable input member and a rotatable output member, both members being rotatable about a common axis. The output member is often in the form of a hub while the input member is a rotatable housing that is hermetically sealed around the hub so as to define a chamber therebetween. Alternately, the input and output arrangements could be reversed in other applications. Within the chamber are two sets of viscous coupling plates. The first set of plates, the inner viscous coupling plates, are splined to the hub and rotate therewith. Similarly, a second set of plates, the outer viscous coupling plates, are splined to the housing and rotate therewith. The outer plates are interleaved with the inner plates. Filling the chamber or space between the housing and the hub is a high viscosity fluid, such as silicone oil, and, usually, a small amount of gas or air.
In operation, if a slight difference in rotational speed occurs between the input and output members, the relatively low shear rate occurring within the viscous fluid will permit relative slip to occur between the rotating members. However, as the speed differential increases, the viscous shear rate also increases and the fluid works to transmit torque from the input hub to the output housing. As so far described, such viscous couplings are well known in the motor vehicle field.
During initial operation of the viscous coupling, torque is primarily transmitted by the shearing forces resulting from relative rotation of the adjacent viscous coupling plates. However, if continuous relative rotation between the input and output members occurs over a period of time, a torque transmission phenomenon appears. This phenomenon is often referred to as torque progression or "humping". During such a torque progression, the amount of torque transmitted between the input and output members substantially increases without a corresponding increase in relative rotation. More particularly, if the rotational speed differential continues to exist, shearing of the viscous fluid will produce a temperature increase within the chamber. Concomitantly, the viscosity of the fluid decreases which, in turn, cause a predictable digression in the amount of torque being transmitted. However, as the fluid temperature continues to rise, the pressure within the sealed chamber increases for causing a number of the coupling plates to axially shift relative to the input and output members. As the pressure continues to increase, adjacent inner and outer plates are forced into mating or frictional contact with one another which results in torque being transferred by a frictional component in addition to the already present viscous shear component. Once frictional contact has been established between the plates, torque transmission is suddenly and significantly amplified producing the "humping" phenomenon stated above.
Because torque progression is partially caused by the surface to surface frictional contact of adjacent viscous coupling plates, the "humping" phenomenon can be enhanced if the contacting surfaces can be made reasonably free of the viscous fluid. To this end, the contacting surface of one viscous coupling plate can be "wiped" substantially free of the viscous fluid by the contacting surface of an adjacent viscous coupling plate.
Viscous coupling plates are generally manufactured via a stamping operation in a machine press from a thin sheet of plate material or stock. The press utilizes male and female dies which cause the coupling plates to conform to the desired configuration. Invariably, a stamped viscous coupling plate exhibits die roll (i.e. a rounded edge) on the edges of one side of the openings formed in the plate and a fine uneven burr on the edges of the opposing side. In operation, the uneven burr is quickly worn from the plate surface by the frictional forces which produce the amplified torque progression. Unfortunately, once the uneven burr has been worn from the plate surface, the viscous fluid introduced onto the plate surfaces by the die roll can not be effectively wiped from the plate surfaces.
Conventionally, die roll has been eliminated on some viscous coupling plates by milling the openings or slots formed therein so as to grind off the die roll. Die roll has also been avoided by replacing the stamping process with laser cutting of the plates themselves. However, both of the above methods have not proven themselves to be cost effective and, in actuality, have proven to be difficult to duplicate and incorporate into a production setting. Another known method for providing a wiping edge is to bend the edge of the viscous clutch plate as disclosed in U.S. Pat. No. 4,989,687. However, such bent edges may produce an overly aggressive (i.e. sharp) wiping edge that causes excessive metal wear on the mating plates.
With the above discussion in mind, it is an object of the present invention to overcome the disadvantages associated with conventionally processed viscous coupling plates while enhancing the torque progression characteristics of the viscous coupling apparatus.
It is another object of this invention to manufacture a viscous coupling plate which is configured to substantially remove or "wipe" viscous fluid from the surface of an adjacent coupling plate and thereby enhances frictional contact of the adjacent plates without causing increased wear on the adjacent mating plate.
A further object of the present invention is to produce a viscous coupling plate having a raised "wiping" surface for removing viscous fluid from the planar surface of an adjacent coupling plate which is not readily susceptible to frictional wearing.
Another object of this invention is to produce a viscous coupling plate which exhibits the above advantages and objects and which is cost effective to produce.
In achieving the above objects, the present invention provides for a viscous coupling apparatus which exhibits enhanced torque progression characteristics and which can be cost effectively produced. The viscous coupling plates of the present invention include raised surfaces which are formed by coining the edges of the slots and/or openings formed in the plates. The die roll on the coined edges provides an arcuate wiping surface that exhibits increased frictional wearability over that exhibited by the uneven burr. Also, fabrication of the coupling plate may be accomplished in a cost effective manner in, for example, a progressive die machine press. A first stamping operation generally forms the coupling plate and creates the die roll edges. A coining operation is then readily performed thereafter without removal of the plate from the machine press.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of a preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.